1. Field of the Invention
The present invention relates to a grid-connected power supply system and more particularly to a method for controlling a grid-connected power supply system.
2. Description of the Related Art
The increasing use of energy since industrial revolution has brought with it the concern of global energy depletion that human beings have to encounter sooner or later. Looking for alternative energy has become a worldwide urgent matter for mankind to keep developing for now and in the future. Generating electricity from solar energy to supply power to electric appliances is a rather commonplace technique currently.
Suppliers of conventional grid-connected power supply system have begun to incorporate converted alternative energy, such as wind energy, solar energy and the like, with the utility grid to reduce the mains power consumption. With reference to FIG. 4, a conventional grid-connected power supply system has a first power supply module 80 and a second power supply module 90. The first power supply module and the second power supply module absorb solar energy and convert it into electricity through two respective solar panels 81, 91. Electricity is further converted into DC power by two DC to DC converters 82, 92 and two rectifiers 83, 93 to store the DC power in two energy storage capacitors C11, C22. After a stored voltage Vbus1 or Vbus2 of the energy storage capacitor C11, C22 is greater than the voltage of the post-end mains power, a relay 70 closes and DC power outputted from the rectifiers 83, 93 is converted into AC power through two DC to AC converters 84, 94 and then outputted to the output capacitors C1 and C2 and further to the mains power grid. Hence, the regenerative energy, such as solar energy, wind energy and the like, is converted into electricity, which is further converted into AC power and outputted to the mains power grid, so as to reduce mains power consumption.
However, when one of the solar panels 81, 91 fails to supply solar energy to the first and second power supply modules 80, 90 and the stored voltage Vbus1, Vbus2 of the energy storage capacitors C11, C12 is less than a peak voltage value of the mains power, the voltage of the mains power charges the energy storage capacitors C11, C12 through a body diode of each metal oxide semiconductor field effect transistor (MOSFET) inside the DC to AC converters 84, 94. A surge current is generated during the charge-back process to pass through the body diode of each MOSFET and discharge huge power heating up or even burning out the body diode of each MOSFET. To prevent the mains power from charging the energy storage capacitors C11, C12, with reference to FIG. 5, a relay 85, 95 is usually connected to an output terminal of each of the first and second power supply modules 80, 90 to ensure that each of the first and second power supply modules 80, 90 is isolated from the mains power to avoid further damaging the DC to AC converters 84, 94.
Although the approach connecting the relay to each of the first and second power supply modules 80, 90 indeed isolates each of the first and second power supply modules 80, 90 and further prevents the DC to AC converter 84, 94 inside each of the first and second power supply modules 80, 90 from being burned out, the relay is costly and occupies more space due to its bulky size. In the case of a power supply system having multiple sets of power supply modules, the cost and size issues even aggravate.